Black Hole's Spin Revealed for 1st Time
I think it should have done if it's in Nature. The peer review process determines its suitability for publication. It happens prior to publication.
So, the Black Hole gave an interview blaming gravitic effects on Superstrings? That's spin, alright!
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Well, here's a bit of an issue when you say they have "confirmed" the spin on a black hole. Here I put on my pedantic hat.
You can't observe the hole directly. For a black hole, we expect an, "innermost stable orbit". Get any closer, and matter can't be in a long-term orbit around the hole - it is either falling in, or escaping. Hanging around is not an option. The math says that, if the hole is spinning, it twists space in a particular way, and that leads to an ISO that is rather closer than for a non-spinning hole.
My quick read of secondary sources (I haven't read the Nature article, yet) goes thusly: They got a good look at the accretion disk around a super-massive black hole*. Measuring how far that is from the hole, they assume the inner-edge of the disk is at the innermost stable orbit, and thus derive how quickly the hole is spinning. Simple enough.
But, "confirmation" generally requires that you have *two* ways to measure something, and they agree. One measurement, using a theoretical construct as to the measurement's meaning, is not "confirmation" of that meaning.
If we accept the measurement, though, it does have implications for how these holes come to be. If you expect most of the growth is from random infalling material, then you expect the hole's spin to decrease as it grows - the infalling matter, coming in at random angles, will overall add no angular momentum. Adding mass but no momentum over time means you're generally slowing the spin. But, this bugger is spinning *fast* - at about 84% of the maximum allowed by relativity**.
This implies that the hole grew in some sort of ordered way or process - in general, stuff falling in added to its angular momentum, rather than falling in randomly as we'd expect.
*Not directly - it was a complicated thing, that includes determining the red-shift of X-rays coming off the heated gas of the disk.
**You'll see reports that it is spinning "at 84% of the speed of light". That's a bit misleading, as there's no hard surface or particle on the hole (that we can see) that is physically moving at all.
You can't observe the hole directly. For a black hole, we expect an, "innermost stable orbit". Get any closer, and matter can't be in a long-term orbit around the hole - it is either falling in, or escaping. Hanging around is not an option. The math says that, if the hole is spinning, it twists space in a particular way, and that leads to an ISO that is rather closer than for a non-spinning hole.
My quick read of secondary sources (I haven't read the Nature article, yet) goes thusly: They got a good look at the accretion disk around a super-massive black hole*. Measuring how far that is from the hole, they assume the inner-edge of the disk is at the innermost stable orbit, and thus derive how quickly the hole is spinning. Simple enough.
But, "confirmation" generally requires that you have *two* ways to measure something, and they agree. One measurement, using a theoretical construct as to the measurement's meaning, is not "confirmation" of that meaning.
If we accept the measurement, though, it does have implications for how these holes come to be. If you expect most of the growth is from random infalling material, then you expect the hole's spin to decrease as it grows - the infalling matter, coming in at random angles, will overall add no angular momentum. Adding mass but no momentum over time means you're generally slowing the spin. But, this bugger is spinning *fast* - at about 84% of the maximum allowed by relativity**.
This implies that the hole grew in some sort of ordered way or process - in general, stuff falling in added to its angular momentum, rather than falling in randomly as we'd expect.
*Not directly - it was a complicated thing, that includes determining the red-shift of X-rays coming off the heated gas of the disk.
**You'll see reports that it is spinning "at 84% of the speed of light". That's a bit misleading, as there's no hard surface or particle on the hole (that we can see) that is physically moving at all.
freyar
Extradimensional Explorer
I have a slightly different take than Umbran, one which is both slightly more optimistic and also more jaded.
The optimistic part: The way they measure the spin as Umbran describes is actually a lower limit, since the glowing accretion disk can't extend
farther in than the innermost stable orbit. So if it really doesn't go that far, they've underestimated the spin. The big assumption here, of course, is that Einstein's general relativity is right for gravity around this kind of black hole, and every experiment done so far agrees that it should be. Also, like Umbran, I haven't read the Nature article (no subscription at home), but it sounds from the other links that this team also measured other relativistic distortions of the glow from the accretion disk, which provides another handle on what the spacetime is near the black hole (and therefore the spin). I don't have any idea of the details, though.
The jaded part: As I just happened to post a few weeks ago, this isn't actually the first black hole to have its spin measured. Here's an article from 2006 about another one. It's possible that the new Nature article is the first to do some aspect of the measurement, but the press release is certainly putting a "spin" on things, if you'll pardon the pun.
The optimistic part: The way they measure the spin as Umbran describes is actually a lower limit, since the glowing accretion disk can't extend
farther in than the innermost stable orbit. So if it really doesn't go that far, they've underestimated the spin. The big assumption here, of course, is that Einstein's general relativity is right for gravity around this kind of black hole, and every experiment done so far agrees that it should be. Also, like Umbran, I haven't read the Nature article (no subscription at home), but it sounds from the other links that this team also measured other relativistic distortions of the glow from the accretion disk, which provides another handle on what the spacetime is near the black hole (and therefore the spin). I don't have any idea of the details, though.
The jaded part: As I just happened to post a few weeks ago, this isn't actually the first black hole to have its spin measured. Here's an article from 2006 about another one. It's possible that the new Nature article is the first to do some aspect of the measurement, but the press release is certainly putting a "spin" on things, if you'll pardon the pun.
trancejeremy
Adventurer
It's been 20 years since my last Astrophysics class. But I seem to recall that one of the interesting properties of a rotating black hole is that it no longer has a point singularity, but a ring shaped one.
freyar
Extradimensional Explorer
Yes, that's right, assuming general relativity. Of course, near a singularity, we expect GR to be replaced with some more complete theory of (quantum) gravity.
I saw a bit more about this at the BBC. It seems the claim is that this is the first spin measurement of a supermassive black hole of the type found in centers of galaxies like our own and does use some new techniques. In any case, it's a very important measurement since it gives us some information about the formation of those black holes (and probably galaxies), but it does seem at least that some of the press has gotten a bit carried away with things.
They heard there was news about a black hole and just got sucked in...
The singularity, by relativistic definition, has no volume, correct. And the Event horizon is not made of matter, or anything like that. However, there is still spin.
There are a few things in this Universe that cannot be created or destroyed, and may only be transformed. Energy is one of them. Momentum is another. When we say that something is spinning, that means it has some rotational kinetic energy, some angular momentum. When a star or other mass collapses into a black hole, that energy and momentum have to go *somewhere*. Either the mass has to shed it into the universe before it collapses, or it keeps it after collapse. If it keeps it, well, then whatever the volume of the singularity, it sill has to be spinning, by virtue of the fact that stopping isn't an option.
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